Chromate process



United States Patent 3,159,509 .CHROMATE PROCESS Rihei Tomono, Ukyo-hu,Kyoto, Japan, assignor to Ohuno Chemical Industry Company, Osaka, .lapanNo Drawing;

1 Claim. (Cl. 148--6.16)

This invention relates to an improvement in the chromate process whichis employed in the art for elevating the corrosion resistance ofarticles electroplated with zinc or cadmium and of zinc die castings.More particularly, the invention pertains to a method wherein a salt oftitanium is dissolved in the chromate bath and the article to be treatedis dipped in the bath.

According tothe chromate process employed in the art, the article to betreated is dipped atroom temperature or at elevated temperatures in abath containing a hexavalent chromium compound and an organic orinorganic acid to form a chromate deposit on the surface of the article.The chromium compound employed, for example, can be chromic acidanhydride, potassium bichromate, sodium bichromate and the like. Detailsof the process are disclosed in Electroplating Engineering Hand book(Reinhold Publishing Corp., 1955, pp. 366 to 376), and the property andcomposition of the chromate deposit in L. F. Spencer, Metal Finishing(vol. 158, No. 1, pp. 58 to 65), W. E. Pocock, Metal Finishing Guidebook(1960 pp. 512 to 524), and S. L. Sisler, 1'. Bass & M. A. Henderson,Plating (vol. 41, No. 2, p. 147). Generally speaking, the integralcomponents of the bath are C10 or an alkali bichromate and sulfuricacid. The article to be treated is dipped in the bath at 20 to 65 C. for3 to 5 seconds. The article is then suspended in the air at roomtemperature for a short period of time, washed with water and dried.

The improved process of this invention comprises dissolving one of thetitanium salts, titanous phosphate, titanous sulfate, titanic sulfateand titanyl sulfate in the ordinary chromate bath, and dipping thearticle to be treated in the resultant bath in an identical manner asemployed in the conventional chromate art. There are no specific changesmade in the composition of the starting chromate bath. The pH of thebath is kept at below 1.5 in order that the desired amount of thetitanium salt satisfactorily dissolves in the bath. A bath containing 80to 400 grams per liter of chromic acid and 5 to 60 grams per liter ofsulfuric acid is expedient. The titanium compound employed in thisinvention can be titanous phosphate, titanous sulfate, titanic sulfateand titanyl sulfate. The most desirable amount of titanium saltdissolved in the bath ranges from 0.5 to 15 grams per liter. 30 gramsper liter or more of titanium salt damage the adhesive power of thedeposit.

The principle of this invention is put into practice in the followingmanner:

A zinc-plated article, for instance, is dipped in the acidic bath ofthis invention containing hexavalent chromium salt and titanium salt.Then the zinc contained in the deposit starts being oxidized anddissolving into the bath, allowing the zinc still remaining undissolvedin the deposit to be sufficiently activated and replacedelectrochemically with the titanium salt, yielding a layer of titanium,probably of titanium hydroxide, on the surface of the zinc deposit. Thehexavalent chromium salt in oxidizing the zinc deposit, on the otherhand, is reduced to trivalent chromic oxide and deposits in the state ofa Filed Nov. 27, 1961, Ser. No. 155,179

3,159,509 Patented Dec. 1, 1964 ice gel on the titanium layer, absorbingtitanium salt, hexavalent chromium salt and acid used in the chromatebath. The article is then suspended in the air at room temperature forabout 5 seconds and dried at 50 to 80 C. The layer or deposit thusobtained is hard and compact, and the brilliancy and corrosionresistance are markedly improved as compared with the articleconventionally chromated.

The metals which can be treated directly or indirectly with the methodof this invention include iron, zinc, cadmium, copper, brass, aluminum,bronze, silver, magnesium and the like.' Said metals can all be treatedin the same manner as described above, yielding a satisfactory and muchimproved corrosion resistance, although the replacement with thetitanium salt in the bath does not take place with the metals havinghigher electrochemical activity than titanium.

To have a better understanding of this invention, preferred examples ofthe same are disclosed below:

Example 1 A soft iron article (10 cm. x 10 cm. x 0.04 cm.) is elec- 3trochemically plated with zinc according to the conventi onal method.More particularly, 42 g./l. of ZnO, 92 g./l. of NaCN and 40 g./l. ofNaOH are dissolved in water. Electric current is applied in the order of3 amp./

dm. in terms of cathode current density, and the article is dipped inthe bath at 18 to 20 C. for 25 to 30 minutes to be electrochemicallyplated with a zinc deposit 20 microns thick. The article is then Washedwith Water, dipped for 2 seconds in an aqueous solution containing 0.5weight percent of nitric acid, and dried in the air.

The zinc-plated article is then dipped at 20 to 30 C. for 5 seconds inthe bath of this invention containing 200 g./l. of CrO 10 g./l. of TiOSO-2H O and 20 g./l. of H 80 The article thus treated is suspended in theair for 5 seconds, washed with water, and dried in the air at C. for 5to 6 hours. As compared to a control article which is prepared in anidentical manner as described above without TiOSO -2H O, the brilliancyof the article treated with the titanium bath of this invention is morepronounced than that of the control article but with no substantialdifference in the color.

To ascertain the presence of titium salt, the sample article is dippedin dilute sulfuric acid to dissolve the titanium layer, and 2 cc. of 3percent hydrogen peroxide are added to the resulting solution andagitated. Then the solution turns yellow in confirmation of the presenceof titanium salt. Control article is also treated in the same manner,but the solution remains unchanged in color.

Sodium carbonate is also employed in amount of 30 g./l. to decolor thelayer, and the layer is subjected to the same test as above, yieldingthe same result.

When subjected to the Cass Test (Metal Finishing, vol. 57, No. 12,(1959)) and Corrodkote Test (Proc. Am. Electroplaters Soc, vol. 43, No.50 1956) the corrosion resistance of the sample article is found 3 to 5times more than that of control. a

The brilliancy of the layer decolored in an alkaline bath is damagedwhen TiOSO -2H O is employed in amount more than 16 g./l. in the bath ofthis invention.

Example 2 A soft iron article of the same size as employed in Example 1is electrochemically plated with cadmium.

More particularly, 32 g./l. of CdO and 75 g./l. of NaCN are dissolved inwater. Electric current is applied in the order of 1 amp./dm. in termsof cathode current density, and the article is dipped in the bath at 18to 20 C. for 50 minutes to be electrochemically plated with a cadmiumdeposit 20 microns thick. The article is then treated with the same bathand in the same manner as employed in Example 1, yielding a uniformtitanium layer containing chromate on the surface of the article. 5percent NaCl is then sprayed on the article to measure the period oftime required for decomposing the deposit into an opaque rust. Findingsare as follows:

It is our intention that the invention be not limited by any of thedetails of description unless otherwise specified, but rather beconstrued broadly within the spirit and scope of the invention as setforth in the the accompanying claim.

What is claimed is:

A method of producing a corrosion resistant coating upon a metal articlehaving a surface selected from the group consisting of zinc and cadmiumwhich comprises dipping such article in an aqueous chromating bath of pHbelow 1.5 consisting of an aqueous solution of 0.5 to grams per liter ofa titanium compound selected from the group consisting of titanoussulfate, titanic sulfate, titanyl sulfate and titanous phosphate and 80to 400 grams per liter of chromic acid, suspending the dipped article inair, Washing the article with water, and drying the article at atemperature between and C.

References Cited by the Examiner UNITED STATES PATENTS 2,786,002 3/57Van et a1 l486.2 2,798,830 7/57 Newhard et a1. l486.2 2,851,385 9/58Spruance et al. 1486.2

RICHARD D. NEVIUS, Primary Examiner.

